The lightweight GAF
Compact Folding viewer was supplied folded flat in the back of some
medical reference books published by Mosby in the mid 1970's. The example
show below was from a book that was published in 1974 and also came with
16 medical reels.

The design of the
viewer tends to suggest that it probably wasn't intended to be folded and
unfolded many times, but rather that it be bent into position, clipped
together and then left assembled. The viewer doesn't have hinges, but
rather relies on the plastic being bent to form the required shape. Over
flexing of the plastic could ultimately lead to the viewer snapping
through fatigue of the plastic.